# The sum of the di

Let ab be the 2-digit number.

Given that a + b = 11 and ab – ba = 9

In general, a 2-digit number ab made of digits a and b is equal in value to 10 × a + b = 10 a + b

Similarly, we have ba = 10 × b + a = 10b + b

From the problem statement, we know ab – ba = 9

(10a + b) – (10b + a) = 9

9a – 9b = 9

a – b = 1

So, we have a + b = 11 and a – b = 1. Adding these,

(a + b) + (a – b) = 11 + 1

2a = 12

a = 6

Using a – b = 1, we have b = a – 1

b = 6 – 1 = 5

So, ab = 65

Therefore, the number is 65.

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